The present invention relates to an electronic control system for an automatic vehicle transmission, and more particularly to an electronic control system for controlling a vehicle transmission based on a vehicle speed, an amount of depression of an accelerator pedal, or the like.
FIG. 8 of the accompanying drawings schematically shows an automobile incorporating a conventional electronic control system 1 for automatically controlling a vehicle transmission.
Denoted in FIG. 8 at 2 is an engine, 3 a clutch, 4 a transmission, 5 a final drive unit, 6 a wheel, 7 a selector lever, and 8 an accelerator pedal. Designated at 9 is an input shaft sensor for detecting the rotational speed of an input shaft of the transmission 4, 10 a vehicle speed sensor for detecting the speed of the vehicle, 11 a gear position sensor for detecting a gear position of the transmission 4, and 12 an accelerator sensor for detecting the amount or depth to which the accelerator pedal 8 is depressed. Output signals from these sensors are supplied to the electronic control system 1. The clutch 3 and the transmission 4 are associated with a clutch actuator 13 and a transmission actuator 14, respectively, for mechanically operating the clutch 3 and the transmission 4. The clutch and transmission actuators 13, 14 are controlled by the electronic control system 1 based on the output signals from the sensors 9 through 12 and an output signal from the selector lever 7. A battery 15 serves as a power supply for various electric components on the vehicle and the electronic control system 1.
FIG. 9 shows a block diagram of the electronic control system 1 for explaining main operation thereof. The electronic control system 1 includes a pulse input unit 104 for counting pulses from the vehicle speed sensor 10 and an analog input unit 105 for converting an analog output from the accelerator sensor 12 to a digital signal through A/D conversion. In a processor 106, a count from the pulse input unit 104 is put in a variable "SPEED", and a digital signal from the analog input unit 105 is put in a variable "ACCEL". The processor 106 searches a predetermined gear change map for an optimum gear position based on "SPEED" and "ACCEL". The gear change map is stored in a ROM (not shown) and has a horizontal axis representing the variable "SPEED" and a vertical axis representing the variable "ACCEL". The processor 106 determines the optimum gear position at the intersection of the variables "SPEED" and "ACCEL". The processor 106 then issues the optimum gear position to an output unit 107 which operates the clutch actuator 13 and the transmission actuator 14 for a speed changing operation. After the processor 106 determines the optimum gear position, it operates the clutch actuator 13 to disengage the clutch 3 and then operates the transmission actuator 14 to switch the transmission 4 out of the present gear position into the neutral position and then into the optimum gear position. Thereafter, the processor 106 operates the clutch actuator 13 again to engage the clutch 3, thus completing the gear changing operation.
In FIG. 9, the voltage of the battery 15 is applied via a keyswitch 16 to a power supply 101 and also applied to a backup power supply 103. Even when the keyswitch 16 is open, electric power is supplied from the backup power supply 103 to a data storage RAM (not shown) in the processor 106. A resetting circuit 102 serves to enable the processor 106 to effect an initializing process dependent on a change in the output voltage from the power supply 101.
A processing sequence of the processor 106 will be described with reference to FIG. 10.
An initializing process is effected in a step a.sub.1. In the initializing process, variables such as "SPEED", "SPD:REAL", "ACCEL", and the like, described below are cleared. Then, control goes to a step a.sub.2 in which a count from the pulse input unit 104 is put in the variable "SPD:REAL". A filtering process (digital filter) is effected on the variable "SPD:REAL", which is then put in "SPEED". The filtering process serves to reduce the effect of an error of the vehicle speed data. More specifically, the vehicle speed sensor 10 generates a pulse signal in synchronism with the rotation of the output shaft of the transmission 4. The electronic control system 1 detects the vehicle speed based on the pulse signal from the vehicle speed sensor 10. Generally, the vehicle speed may be detected by either a pulse counting method wherein the vehicle speed is calculated according to the detected period of the pulse signal or a time window method wherein the vehicle speed is calculated according to a count of the pulse signal within a given period of time. The vehicle speed which is determined by these methods tends to fluctuate due to chattering or noise. The adverse effect which the chattering or noise has on the vehicle speed can be removed by the filtering process. Although there are several filtering process types, the period of the pulse signal is determined by averaging the periods of preceding N pulses (N=4) are involved, the average period T is calculated as follows: EQU T=[T(i-3)+T(i-2)+T(i-1)+T(i)].div.4
where T(i-3), T(i-2), T(i-1), T(i) are the periods of the fourth, third, second, and first preceding pulses, respectively, before the time when the periods are averaged.
Thereafter, control proceeds to a step a.sub.3 which refers to the amount of depression "ACCEL" of the accelerator pedal from the analog input unit 105 (FIG. 9) to see if the accelerator pedal has been depressed or not. If depressed, then control goes to a step a.sub.4. In the step a.sub.4, the gear change map is searched for an optimum gear position based on values of "ACCEL" and "SPEED", and the optimum gear position is put in a variable "GEAR". If the accelerator pedal is released, control goes to a step a.sub.6 in which a gear position is determined only from the vehicle speed "SPEED" irrespective of the amount of depression of the accelerator pedal "ACCEL". In this case, no speed change is effected even if the vehicle speed rises since a gear upshift has to be prohibited.
When the optimum gear position "GEAR" is determined, the value of "GEAR" is given to the output unit 107 (FIG. 9) to operate the clutch actuator 13 and the transmission actuator 14. Finally, a step a.sub.7 determines whether an instantaneous power supply cutoff is caused by a hardware error. If not, then control goes back to the step a.sub.2 to repeat the processing from the step a.sub.2 to the step a.sub.7 at a period of 32 msec. If an instantaneous power supply cutoff is developed, then the processor 106 is reset by the resetting circuit 102 in a step a.sub.8 to resume the entire sequence from the initializing step a.sub.1.
FIG. 11(a) shows by way of example a running pattern (relationship between vehicle speeds and time) dependent on a change in the battery voltage (FIG. 11(b)) in the automobile incorporating the conventional electronic control system 1. In the illustrated example, electric power supplied to the electronic control system 1 is instantaneously cut off at a time t.sub.0 by a breakage or short circuit of a vehicle harness, a temporary opening of the keyswitch (FIG. 9), or the like. When this happens, the processor 106 is reset by the steps a.sub.7 and a.sub.8 in FIG. 10 and carries out the initializing process (step a.sub.1). Since "SPD:REAL" and "SPEED" are cleared by the initializing process, an internal value of the processor 106 becomes 0 [km/h] regardless of the fact that the actual speed is a [km/h] as shown in FIG. 11(a).
Subsequently, an optimum gear position is determined in approximately 32 msec. If the accelerator pedal is depressed at this time, the optimum gear position "GEAR" is updated in the step a.sub.4 by the speed change as the vehicle speed increases (i.e., as the value "SPEED" increases). If the accelerator pedal is released, the optimum gear position is determined solely from "SPEED" in the step a.sub.5, and no gear upshift is performed even if the value "SPEED" increases afterwards. In the latter case, as shown in FIG. 11(a), after the time t.sub.0, "SPEED" (indicated by the dot-and-dash line) approaches the actual vehicle speed (indicated by the solid line) at a smaller gradient (because of the filtering process) than "SPD:REAL" (indicated by the broken line). When a gear position is determined at a time t.sub.1, for example, or if the vehicle speed possible in the second gear position is c [km/h], then the optimum gear position is determined as the second gear position based on "SPEED" at the time t.sub.1. When the gears are actually shifted into the optimum gear position at a later time t.sub.2, the optimum gear position remains as the second gear position determined at the time t.sub.1 even if the actual speed is d [km/h] which is far higher than c [km/h]. Therefore, when the gears are actually shifted into the second gear position, the engine overruns causing the vehicle to operate erratically.